1. Technical Field
This invention relates to the art of AM radio broadcasting and, more particularly, to an RF power amplifier system of the type employed in AM radio broadcasting together with improved driver circuitry employing inductive steering.
2. Description of the Prior Art
The U.S. patents to H. I. Swanson U.S. Pat. Nos. 4,580,111 and 4,949,050 disclose an amplitude modulator for use in AM radio broadcasting and wherein the modulator serves to generate an amplitude modulated signal by selectively turning on and off a plurality of RF amplifiers in a digital manner to produce amplitude modulation. Each of the RF amplifiers includes a plurality of switching transistors, each of which may take the form of a MOSFET transistor, connected together in a bridge circuit. This bridge circuit provides output signals to an output combiner. Each of the MOSFET transistors has a gate which is driven by properly phased RF frequency signals that allow the proper MOSFET transistors to be turned on at the correct times.
The drive system for driving the RF amplifier MOSFET switching transistors includes a transformer having a secondary winding for driving each MOSFET switching transistor. This provides a low impedance source of drive for the gate of each MOSFET switching transistor. This also provides the correct out-of-phase drive to the MOSFET switching transistors. Thus, the bridge arrangement includes upper MOSFET switching transistors and lower MOSFET switching transistors. The correct out-of-phase drive to the MOSFET transistors provides the proper gate voltage with respect to the source voltage.
With the onset of digital radio operations, a direct drive operation of the switching transistors is desirable. Such a circuit has been disclosed in the U.S. patent to J.N. Malec U.S. Pat. No. 5,612,647. The present invention is directed towards improvements over those shown in the Swanson patents and the Malec patent.
An inductive steering drive circuit is located in the gate circuit of each of the MOSFET switching transistors. This permits a higher switching frequency and, in addition, eliminates the need for an RF drive tuning circuit frequently employed in amplifiers including transformers such as those disclosed in the Swanson patents referred to above.
Another feature of the present invention is directed toward a direct drive RF power amplifier system that employs a MOSFET buffer amplifier in the gate circuit of each MOSFET switching transistor and wherein the power supply for the buffer amplifiers includes a DC to DC converter operating in synchronism with the carrier frequency employed so that the DC supply voltage for the buffer amplifiers will have a DC ripple voltage which is same as the amplifier carrier frequency. This will minimize intermodulation products because the ripple frequency is locked to the carrier frequency.
Another feature of the present invention is circuitry for synchronously controlling the amplifier in such a manner to insure that the TURN ON and TURN OFF of the amplifier corresponds with the start and end of an RF cycle respectively. Such synchronous operation minimizes damage to the MOSFETs. If the switching is not synchronized, the MOSFETs can be damaged because of a high dv/dt secondary breakdown of the transistors.